Test workflow improvements

- document the Rubies we are using for tests
- document the need for `bundler` gem
- commit `package-lock.json` file for reproducible tests
- switch to Poltergeist for headless tests
- test for existence of socket file
- ease the requirement for tempfile location

README.md

@@ -50,7 +50,7 @@ Pageflow assumes the following choice of libraries:
 
 Pageflow runs in environments with:
 
-* Ruby >= 2.1
+* Ruby >= 2.1 (see `.travis.yml` for supported versions)
 * Node >= 6.9.5
 * Rails 4.2
 * Redis server (for Resque)

doc/contributing/rails_engine_development.md

@@ -5,9 +5,10 @@
 Ensure the development machine meets the requirements listed in the
 readme. From the repository root run:
 
+    $ gem install bundler
     $ bundle install
     $ bin/npm install
-    
+
 Build node package to `assets` directory:
 
     $ bin/npm run build